The present invention relates to black magnetic composite particles for a black magnetic toner and a black magnetic toner using the black magnetic composite particles, and more particularly, to black magnetic composite particles for a black magnetic toner which can exhibit not only an excellent flowability but also a high volume resistivity, and a black magnetic toner using such black magnetic composite particles.
As one of conventional electrostatic latent image-developing methods, there has been widely known and generally adopted a so-called one component system development method of using as a developer, a magnetic toner comprising composite particles prepared by mixing and dispersing black magnetic particles such as magnetite particles in a binder resin, without using a carrier.
The conventional development methods of using one-component magnetic toner have been classified into CPC development methods of using a low-resistance magnetic toner, and PPC development methods of using a high-resistance magnetic toner.
In the CPC methods, the low-resistance magnetic toner used therefor has an electric conductivity, and is charged by the electrostatic induction due to electric charge of the latent images. However, since the charge induced on the magnetic toner is lost while the magnetic toner is transported from a developing zone to a transfer zone, the low-resistance magnetic toner is unsuitable for the PPC development method of using an electrostatic transfer method. In order to solve this problem, there have been developed the insulated or high resistance magnetic toners having a volume resistivity as high as not less than 10.sup.12 .OMEGA..multidot.cm.
Recently, with the high image quality such as high image density or high tone gradation, or with the high copying speed of duplicating machines, it has been strongly demanded to further enhance characteristics of the insulted or high-resistance magnetic toners as a developer, especially a fluidity thereof.
It has been strongly desired that the insulated or high-resistance black magnetic toners are improved in flowability in order to obtain copies which are free from unevenness of developed images, and show a high definition and an excellent gradation.
With respect to such demands, in Japanese Patent Application Laid-Open (KOKAI) No. 53-94932(1978), there has been described "these high-resistance magnetic toners are deteriorated in fluidity due to the high electric resistance, so that there arises such a problem that non-uniformity of developed images tend to be caused. Namely, although the high-resistance magnetic toners for PPC development method can maintain necessary charges required for image transfer, the magnetic toners are frictionally charged even when they are present in other steps than the transfer step, where the magnetic toners are not required to be charged, e.g., in a toner bottle or on the surface of a magnetic roll, or also slightly charged by mechano-electrets during the production process of these magnetic toners. Therefore, the magnetic toners tend to be electrostatically agglomerated, resulting in deterioration of fluidity thereof", and "It is an another object of the present invention to provide a high-resistance magnetic toner for PPC development method which is improved in fluidity, can be prevented from causing non-uniformity of developed images, and has an excellent image definition and tone gradation, thereby obtaining high-quality copies by indirect copying methods".
In recent years, with the reduction in particle size of the insulated or high-resistance magnetic toners, it has been increasingly desired to enhance the fluidity thereof.
With respect to such a fact, in "Recent Electrophotographic Developing System and Comprehensive Data Collection for Development and Utilization of Toner Materials" published by Japan Scientific Information Co., Ltd. (1985), page 121, there has been described "With extensive development of printers such as ICP, a high image quality has been required. In particular, it has been demanded to develop high-precision or high-definition printers. In Table 1, there is shown a relationship between definitions obtained by using the respective toners. As is apparent from Table 1, the smaller the particle size of wet toners, the higher the image definition is obtained. Therefore, when a dry toner is used, in order to enhance the image definition, it is also required to reduce the particle size of the toner . . . As reports of using toners having a small particle size, it has been proposed that by using toners having a particle size of 8.5 to 11 .mu.m, fogs on a background can be improved and toner consumption can be reduced, and further by using polyester-based toners having a particle size of 6 to 10 .mu.m, an image quality, a charging stability and lifetime of the developer can be improved. However, when such toners having a small particle size are used, it has been required to solve many problems. There are problems such as improvement in productivity, sharpness of particle size distribution, improvement in fluidity, etc.".
Further, insulated or high-resistance black magnetic toners widely used at the present time, have been required to show a high degree of blackness and a high image density for line images and solid area images on copies.
With respect to this fact, on page 272 of the above-mentioned "Recent Electrophotographic Developing System and Comprehensive Data Collection for Development and Utilization of Toner Materials", there has been described "Powder development is characterized by a high image density. However, the high image density as well as the fog density as described hereinafter, greatly influences image characteristics obtained".
Further, it is necessary that the insulated or high-resistance black magnetic toners can retain a charge amount required for the development of latent images, as described above. Therefore, it has also been strongly desired that the insulated or high-resistance black magnetic toners have a volume resistivity as high as not less than 10.sup.12 .OMEGA..multidot.cm.
With respect to this fact, in Japanese Patent Application Laid-Open (KOKAI) No. 54-139544(1979), it has been described that "Generally, in electrophotographic copying apparatuses of PPC (plain paper copy) type, when a magnetic toner is used as a developer for developing electrostatic latent images, the use of a magnetic toner having a lower electrical resistance is preferred to neutralize the charge on the electrostatic latent images upon development thereof. On the other hand, upon transfer of the developed images, the use of a higher-resistance magnetic toner is preferred to obtain a good transfer efficiency and sharp images. That is, the characteristics of the magnetic toner required for a good developability, conflict with those for a good transfer efficiency. Accordingly, in order to satisfy both the developability and the transfer efficiency, it is, as a matter of course, necessary to restrict the electrical resistance of the magnetic toner in a specific range. Namely, it is preferred that the electrical resistance of the magnetic toner is usually 10.sup.12 to 10.sup.14 .OMEGA..multidot.cm. Thus, it is known that when the electrical resistance of the magnetic toner lies within such a specific range, it is possible to obtain good results concerning both developability and transfer efficiency".
There is a close relationship between characteristics of the insulated or high-resistance black magnetic toners and properties of magnetite particles which are mixed and dispersed in the black magnetic toner to impart magnetism to the toner, and serve as a black colorant.
That is, since the flowability of the black magnetic toner largely depends upon surface conditions of the magnetite particles exposed to the surface of each black magnetic toner particle, it has been strongly required that the magnetite particles themselves have an excellent flowability.
The blackness and density of the black magnetic toner also largely depend upon those of the magnetite particles contained in the black magnetic toner. Accordingly, in order to obtain a black magnetic toner having an excellent blackness, the magnetite particles are usually required to be contained in the black magnetic toner in an amount of about 30 to about 50% by weight.
As described above, the insulated or high-resistance black magnetic toner is required to have an insulating property enough to retain a necessary charge amount thereon, especially show a volume resistivity of not less than 10.sup.12 .OMEGA..multidot.cm. However, the black magnetic toner usually contains pigments such as carbon black, dyes, charge-controlling agents, etc., in addition to a binder resin and magnetic particles such as magnetite particles, resulting in reduction in charge amount of the black magnetic toner.
With respect to this fact, on pages 46 to 47 of "Optimum Design of Developers and Developing Process Techniques in Electrophotgraphy" published by Technical Information Institute, Co., Ltd. (1994), it has been described that "In developers of a contact-charging type, it is necessary that at least one of carrier and toner has an insulating property enough to retain the charge amount for such a period of time as required for the development. In general, since the toner image is required to have an electrostatic transfer property and a heat- (or pressure-) fixing property, it is preferred that the toner has an insulating property rather than the carrier (therefore, there exist upper limits concerning the mixing ratios of conductive pigments, carbon, magnetite, etc., which are mixed in the toner). Ordinary toners contain not only simple polymers but also other components such as pigments (e.g., carbon black), dyes, charge-controlling agents or the like. The charge amount of toner is usually reduced by adding conductive fine particles, carbon black or Fe.sub.3 O4 thereto. It is suspected that the reduction in charge amount of the toner is caused by a microscopic charge-removing effect at the contact portion".
Accordingly, in order to obtain a black magnetic toner having a volume resistivity as high as possible, it has been strongly desired to increase a volume resistivity of magnetite particles as highly as possible, which are contained in the toner in a large amount and normally have a low volume resistivity, especially about 1.0.times.10.sup.6 to 5.0.times.10.sup.7 .OMEGA..multidot.cm.
On the other hand, various attempts have been made in order to improve a flowability of the black magnetic toner. For example, there have been known a method of adhering SiO.sub.2 fine particles onto the surfaces of magnetite particles mixed and dispersed in black magnetic toner (Japanese Patent Application Laid-Open (KOKAI) Nos. 2-73362(1990) and 6-130719(1994), etc.), a method of exposing a silicon compound to the surfaces of magnetite particles mixed and dispersed in black magnetic toner (Japanese Patent Publication (KOKOKU) No. 8-25747(1996), etc.), and the like.
Further, in order to improve a dispersibility of magnetite particles mixed and dispersed in black magnetic toner, it have also been known a method of treating the surfaces of the magnetite particles with an organosilicon compound such as methyl hydrogen polysiloxane (Japanese Patent Application Laid-Open (KOKAI) Nos. 3-43748(1991) and 53-81125(1978), etc.), and the like.
Thus, it has been most strongly demanded to provide black magnetic composite particles for black magnetic toner which have not only an excellent flowability but also a high volume resistivity. However, black magnetic toners which can satisfy such properties have not been provided.
That is, in the case of any of the above-mentioned conventional magnetite particles which have aimed at improving a flowability of the black magnetic toners, the SiO.sub.2 fine particles adhered thereon tend to be fallen-off or desorbed from the surface of each magnetite particle when these magnetite particles are dispersed in a binder resin, as described in Comparative Examples hereinafter, so that the black magnetic toners cannot show a sufficient flowability. In addition, these magnetite particles have a volume resistivity as low as about 10.sup.6 to about 10.sup.7 .OMEGA..multidot.cm, as described in Comparative Examples hereinafter. Further, as also described in Comparative Examples hereinafter, the magnetite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 3-43748(1991) or the like, have not been improved in flowability, and the volume resistivity thereof is insufficient, i.e., about 10.sup.7 .OMEGA..multidot.cm at most.
As a result of the present inventor's earnest studies for solving the above problems, it has been found that by causing fine particles comprising oxides and/or oxide hydroxides of at least one element selected from the group consisting of Si, Zr, Ti, Al and Ce, to adhere or exist on at least a part of the surface of each magnetite core particle, and then coating the surface of the fine particles adhered or existing on the surface of each magnetite particle or the surface of the fine particles adhered or existing on the surface of each magnetite particle and the exposed surface of each magnetite core particle, with methyl hydrogen polysiloxane, the obtained magnetic composite particles can show not only an excellent flowability, but also have a high volume resistivity. The present invention has been attained on the basis of the finding.